1. Field of the Invention
This invention relates to a process for the preparation of 1,5-dimethyl-1,5-cyclooctadiene and 1,4-dimethyl-4-vinyl-1-cyclohexene from isoprene.
2. Description of the Prior Art
1,5-Dimethyl-1,5-cyclooctadiene is an intermediate in the synthesis of Vitamin E, see British patent applications Nos. 49796/74 and 5091/75, the aroma chemicals 7-hydroxy-3,7-dimethyl-1-o-octene (dihydromyrcenol), see British patent application No. 4371/76, and alphalinalool and linalool, see British patent application No. 45309/76.
1,4-Dimethyl-4-vinyl-1-cyclohexene is an intermediate in the synthesis of aroma chemicals having characteristic floral odors, see British patent application No. 33050/76, and woody odors, see British patent application No. 7818/76.
1,5-Dimethyl-1,5-cyclooctadiene was formed by contacting isoprene with a mixture obtained by combining a solution of nickel acetylacetonate and triphenyl phosphine in benzene with a solution of ethoxydiethylaluminum in benzene, see Example 61 of French patent specification No. 1,283,217. A disadvantage of this known process is that a mixture of 70% 1,5-dimethyl-1,5-cyclooctadiene and 30% 1,6-dimethyl-1,5-cyclooctadiene was formed. As 1,6-dimethyl-1,5-cyclooctadiene is not an intermediate in the synthesis mentioned hereinbefore it should be separated from the 1,5-dimethyl-1,5-cyclooctadiene. However, such a separation of these materials is very difficult, their boiling points being close to each other. Therefore, a process in which hardly any 1,6-dimethyl-1,5-cyclooctadiene is formed, if at all, would be very attractive.
U.S. Pat. No. 3,436,431 issued Apr. 1, 1969, to Candlin et al, U.S. Pat. No. 3,522,321 issued July 28, 1970, to DeYoung, Chem. Abstracts 60 (1964) 11883e, Chem. Abstracts 66 (1967) 28422a and Chem. Abstracts 80 (1974) 120396g are references which disclose the dimerization of isoprene in the presence of a complexed group VIII metal. These references do not however use the particular complexing agent of this invention, nor is the content of 1,6-dimethyl-1,5-cyclooctadiene maintained at as low levels as in the instant process.